From 6691202998cc332506b94e3d2487ca3e4996569a Mon Sep 17 00:00:00 2001
From: Bagatur <22008038+baskaryan@users.noreply.github.com>
Date: Wed, 30 Oct 2024 16:56:47 -0700
Subject: [PATCH] anthropic[patch]: allow multiple sys not at start (#27725)

---
 .../langchain_anthropic/chat_models.py        | 15 ++++++++-----
 .../tests/unit_tests/test_chat_models.py      | 22 +++++++++++++++++++
 2 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/libs/partners/anthropic/langchain_anthropic/chat_models.py b/libs/partners/anthropic/langchain_anthropic/chat_models.py
index c4fb0b1775..c408cdc23d 100644
--- a/libs/partners/anthropic/langchain_anthropic/chat_models.py
+++ b/libs/partners/anthropic/langchain_anthropic/chat_models.py
@@ -139,7 +139,10 @@ def _merge_messages(
                     ]
                 )
         last = merged[-1] if merged else None
-        if isinstance(last, HumanMessage) and isinstance(curr, HumanMessage):
+        if any(
+            all(isinstance(m, c) for m in (curr, last))
+            for c in (SystemMessage, HumanMessage)
+        ):
             if isinstance(last.content, str):
                 new_content: List = [{"type": "text", "text": last.content}]
             else:
@@ -148,7 +151,7 @@ def _merge_messages(
                 new_content.append({"type": "text", "text": curr.content})
             else:
                 new_content.extend(curr.content)
-            merged[-1] = curr.model_copy(update={"content": new_content}, deep=False)
+            merged[-1] = curr.model_copy(update={"content": new_content})
         else:
             merged.append(curr)
     return merged
@@ -174,14 +177,14 @@ def _format_messages(
     merged_messages = _merge_messages(messages)
     for i, message in enumerate(merged_messages):
         if message.type == "system":
-            if i != 0:
-                raise ValueError("System message must be at beginning of message list.")
-            if isinstance(message.content, list):
+            if system is not None:
+                raise ValueError("Received multiple non-consecutive system messages.")
+            elif isinstance(message.content, list):
                 system = [
                     (
                         block
                         if isinstance(block, dict)
-                        else {"type": "text", "text": "block"}
+                        else {"type": "text", "text": block}
                     )
                     for block in message.content
                 ]
diff --git a/libs/partners/anthropic/tests/unit_tests/test_chat_models.py b/libs/partners/anthropic/tests/unit_tests/test_chat_models.py
index 859a2fa5ac..b37246e986 100644
--- a/libs/partners/anthropic/tests/unit_tests/test_chat_models.py
+++ b/libs/partners/anthropic/tests/unit_tests/test_chat_models.py
@@ -695,6 +695,28 @@ def test__format_messages_with_cache_control() -> None:
     assert expected_messages == actual_messages
 
 
+def test__format_messages_with_multiple_system() -> None:
+    messages = [
+        HumanMessage("baz"),
+        SystemMessage("bar"),
+        SystemMessage("baz"),
+        SystemMessage(
+            [
+                {"type": "text", "text": "foo", "cache_control": {"type": "ephemeral"}},
+            ]
+        ),
+    ]
+    expected_system = [
+        {"type": "text", "text": "bar"},
+        {"type": "text", "text": "baz"},
+        {"type": "text", "text": "foo", "cache_control": {"type": "ephemeral"}},
+    ]
+    expected_messages = [{"role": "user", "content": "baz"}]
+    actual_system, actual_messages = _format_messages(messages)
+    assert expected_system == actual_system
+    assert expected_messages == actual_messages
+
+
 def test_anthropic_api_key_is_secret_string() -> None:
     """Test that the API key is stored as a SecretStr."""
     chat_model = ChatAnthropic(  # type: ignore[call-arg, call-arg]